Complex metal oxide powders are used as materials for high-performance electronic components such as phosphors, dielectrics, piezoelectrics, ferroelectrics, ferrite, etc. Particularly, titanate powders provide an excellent performance as dielectrics, ferroelectrics, piezoelectrics, and pyroelectrics, etc., and are used in, for instance, multilayer capacitors, actuators, and filters. In response to demands for reducing the sizes of electronic devices, studies for reducing the sizes of these electronic components are promoted, and the complex metal oxide powders as materials have been improved so as to be suitable for the size-reduced electronic components.
For instance, in a multilayer capacitor formed by using barium titanate, which is one of titanates as complex metal oxides, dielectric layers that form a capacitor are made of sintered barium titanate, and one layer is as thin as 10 μm. To achieve this thickness, the barium titanate particles are required to be fine particles with a mean primary particle diameter of not more than 0.3 μm.
Besides, as a sintered products are generally required to have finer grain size to increase the mechanical strength of electronic components, complex metal oxide powders as materials of sintered products are required to have finer particles. In the case where particles have an approximately spherical particle shape, the sintering property is improved. Therefore, complex metal oxide powders having fine particle in an approximately spherical particle shape are demanded.
Conventionally, as a method for producing a complex metal oxide, the solid phase synthesis has been known, in which powders of metal element compounds are mixed and calcined, and an obtained complex metal oxide is crushed. Besides, the wet processes are also known, for instance, the hydrolysis of alkoxide method in which alkoxides of metal elements composing a complex metal oxide is hydrolyzed and precipitates thus obtained are dried and calcined, the hydroxide method in which precipitates obtained by neutralization of an aqueous solution containing metal elements composing a complex metal oxide is dried, calcined, and crushed, and the hydrothermal synthesis method in which an aqueous solution containing compounds of metal elements composing a complex metal oxide is heated under a high pressure.
The solid phase method and the hydroxide method are economical methods suitable for industrial manufacture, but they require an intensive milling step, which has drawbacks of contamination in a powder due to the chipping of a ball or the like used in a ball mill, and fine particles tend to aggregate because of high surface energy.
Therefore, by such a conventional method, it is difficult to produce a complex metal oxide fine powder used as a material for a dielectric layer of a multilayer capacitor that is required to be further thinner.
JP8(1996)-119745A discloses a method for producing a perovskite-structure oxide powder made of barium titanate or the like by the hydrothermal synthesis method. However, since including a heat treatment at 200° C. under a pressure of 20 Kg/cm2, the method requires a specific device, and is not deemed to be a costly advantageous producing method.
JP7(1995)-187612A discloses a method for producing a complex metal oxide powder having a mean primary particle diameter of 1.0 μm to 45 μm and having an approximately spherical particle shape by calcination in a hydrogen halide gas, but from the viewpoint of making particles finer, it is not sufficient yet.
JP8(1996)-208226A discloses a method for producing a complex metal oxide powder having a mean primary particle diameter of 0.3 μm and having an approximately spherical shape by calcination in an iodine atmosphere. The method in which iodine is vaporized for use, however, cannot be considered as a method advantageous for industrial manufacture.
An object of the present invention is to provide a method for producing a complex metal oxide powder whose particles are very fine, each in an approximately spherical particle shape.